It took me some time to figure this one out since I didn't touch
anything near it.

My theory is this:
With COCOON3-69 ehcache was upgraded from 1.6.1 to 2.4.3.
Between those versions, the serialVersionUID of
net.sf.ehcache.Element changed, so serialized versions of that class
cannot be loaded any more.

The build fails because the ehcache for cocoon-profiling is
configured to be "diskPersistent" and reading the old cache with the
new class fails.
The build fails only sometimes, because it fails only once on each
node in the build cluster, after that the cache is rewritten with
the newer version.

Should we wait until the build has spread through all nodes and
stopped failing from this or try something else?
I'm really for changing the profiling cache to non-persistent -
unless that breaks something.
Do we need to preserve profiling results between restarts?